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Two University of Delaware researchers have discovered a means to detect and identify damage within advanced composite materials by using a network of tiny carbon nanotubes, which act in much the same manner as human nerves.

The discovery has important implications in monitoring the health of composite materials used in the construction of a variety of essential products, including commercial airliners.

The research is the work of Tsu-Wei Chou, Pierre S. du Pont Chair of Engineering, and Erik Thostenson, assistant professor of mechanical engineering, and is featured in an article published in Advanced Materials.

Chou said the research team has been working in the field of fibre composites in conjunction with UD’s Center for Composite Materials and of late has taken an interest in the reinforcement of composites with carbon nanotubes.

“Carbon nanotubes are very small but have superb qualities,” Chou said. “They are very light, with a density about one-half that of aluminium, which itself is considered exceptionally light in comparison to other metals, and yet are 30 times as strong as high-strength steel and as stiff as diamonds.”

Besides being very strong and very light, the carbon nanotubes have an incredible ability to conduct heat and electricity. In the latter case, they are 1,000 times more effective at carrying an electrical current when compared to copper.

“Carbon nanotubes have excellent properties and the challenge has been how best to utilize them, to translate those properties into applications,” Chou said.

Given the various properties, Chou and Thostenson set out to develop the carbon nanotubes as sensors embedded within composite materials.

Chou said that the traditional composite materials have weak spots in the interface areas, particularly where there are pockets of resin. As a result, defects, including tiny microcracks, can occur, which over time can threaten the integrity of the composite. Thostenson said the carbon nanotubes can be used to detect defects at onset by embedding them uniformly throughout the composite material as a network capable of monitoring the health of the composite structures.

“Nanotubes are so small they can penetrate the areas in between the bundles of fibre and also between the layers of the composite, in the matrix rich areas,” Thostenson said. Because the carbon nanotubes conduct electricity, they create a nanoscale network of sensors that work “much like the nerves in a human body.”

The researchers can pass an electrical current through the network and if there is a microcrack, it breaks the pathway of the sensors and they can measure the response. Thostenson added that the carbon nanotubes are minimally invasive and just 0.15 percent of the total composite volume.

At present, composite material engineers have limited means to either detect the initial onset of microcracks or identify the specific type of defect. This finding will change that because the method is simple, does not require expensive equipment and is sensitive to the initial stages of microcracking, according to Thostenson.

For the technique to be successful the carbon nanotubes must be scattered everywhere throughout the material and Chou credited his colleague with “developing a technique for disbursing the carbon nanotubes very uniformly in the matrix material.”

Erik Thostenson: “Nanotubes are so small they can penetrate the areas in between the bundles of fibre and also between the layers of the composite, in the matrix rich areas,” Thostenson said.

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